Place egg crate on top of tank. Using a Sharpie or similar marker go around tank perimeter from below and mark tank contour on the egg crate. Using strong cutters or a fine toothed saw go around the marked line and follow contour. Now, that is the outside contour of tank's plastic frame, so you can remove one extra line of material from the back flat side and a bit more from each side. Fit adjust as needed and you are done. Rest of construction follows standard false bottom procedure. Good luck !